Directional acoustic phased arrays can be used in applications, such as aircraft location apparatus that operated as a four-point phased array. Although acoustic detection systems for aircraft are inferior to radar-based detection systems, the principles of acoustic signal focusing and acoustic phased arrays can be applied successfully in other applications.
For example, consider a parabolic microphone where the transducer faces the surface of a parabolic reflector. The shape of a parabola is such that there is a constant time of flight for acoustic signals emitted by a distant source to the surface of the parabolic reflector and then to the transducer.
Given constant time of flight, the different wave pathways have constructive interference and provide a strong signal along an axis of the reflector. Other directions have varying times of flight so the acoustic waves have destructive interference.
The parabola is not the only possible shape for a directional acoustic system. The “shotgun microphone” includes a long tube, often up to a meter long, with holes or slots arranged along its length. The acoustic transducer is mounted at distal end of the tube with respect to the signal source. Acoustic energy approaching the tube enters the slots or the holes and propagates down the tube to the acoustic transducer.
Just as in the case of the parabolic microphone, acoustic energy approaching along the axis of the tube experiences constant and equal time delays no matter through which slot or hole the energy enters, and so experiences constructive interference. Acoustic energy approaching from other directions propagates to the transducer with unequal time delays and experiences destructive interference, and little if any signal is produced by the transducer.
Unfortunately, both the shotgun microphone and the parabolic microphone have a serious shortcoming—physical size. A parabolic microphone is typically a deep dish 40 cm to 1 m in diameter and half that in depth. A shotgun microphone is a long rod, about 3 cm in diameter and a meter or more long. These shapes are difficult to integrate into an office, retail, home, or automotive environment.
It is an object of the current to produce a directional acoustic transducer with a more useful form factor than the parabolic or shotgun microphones, yet with similar or better directionality.
Noise-cancelling microphones typically use two ports through which the acoustic signal enters, one in the front of the sensor, and one in the back, with the microphone's sensor arranged between the ports. These types of microphones are only appropriate when the source is close to the microphone.
U.S. Pat. No. 6,148,089 describes a unidirectional microphone including a microphone unit having a front acoustic terminal, and a rear acoustic terminal, which is provided in a flat-faced surface such as an outer frame of a display panel for computer, includes a unit fitting portion provided on the flat-faced surface for fitting said microphone unit, the top surface of the plane being flat with respect to the top surface of the front acoustic terminal of the microphone unit, a baffle substrate mounted on the side of the front acoustic terminal of the microphone unit to be disposed in the opening surface of the unit fitting portion, and a side acoustic terminal provided about the baffle substrate in communication with the rear acoustic terminal.